The art of particle analysis includes two broad categories of apparatus. On the one hand, analysis of particulates native in liquid has lead to the development of "wet process" particle analyzers which use various techniques to determine certain characteristics of the particulates in liquid suspension. Illustrative of this type of analyzer is that described in U.S. Pat. No. 4,047,814, to V. Westcott wherein particulates suspended in a liquid are caused to "plate out" on a substrate by application of a force field. The force field may be magnetic or electrostatic, for example. Analysis of the particulates is accomplished after fixing them on the substrate and drying preparatory to optical inspection. An illustrative example of the use of the Westcott invention is directed to analysis of metallic wear particles in lubricating oil from an engine.
An alternative technology, that of electromagnetic radiation (i.e., light) obscuration, scattering and diffraction analysis, allows particulates to be analyzed in their native liquid without the need of fixing and drying as required by Westcott. With this technology, a liquid sample with particulates therein is illuminated in an analysis cell with selected light or other radiation, Analysis of the resulting forward or back light scattering, diffraction, or obscuration provides the desired indication of the size distribution of the particulates in the sample.
Of course, some particulates tend to settle out of the liquid, so that stirring devices for the liquids and particulates therein have had to be developed. Typical of this category of particle analysis devices is one by Leeds and Northrop, a unit of General Signal, Inc. For example, a particle analyzer from this company known under the name Microtrac II, as depicted and described in a 1987 brochure, includes as a stirring device a so-call, "particle circulator", which recirculates liquid with particulates therein through an analysis cell. Almost in defense of this stirring device, the particle circulator itself is stated to be free of dead spaces in which particulates could collect or settle out. Also, a common problem with this type of wet process analysis with stirring is stated to be air entrainment where bubbles are created in the carrier liquid and can interfere with the measurement. The particle circulator of the Microtrac II also assertedly solves to some extent the problem of air entrainment in the carrier liquid. Thus, it is seen that the wet process analysis of particulates has its own set of problems, and that a solution to one problem may itself impose its own deficiencies on the art.
The foregoing type of wet process particle analyzer is also applied to measurement of particulates not native to liquid by dispersing the particulates in a dispersant liquid. Of course, this expedient raises the questions of compatibility of the particulates with the dispersant liquid, for example, of wetting the particulates with the liquid, of solution of particulates or constituents thereof in the dispersant, and of agglomeration of the particulates in the liquid, to name just a few concerns. However, the degree to which the problems of wet particulate analyzers are brought to the measurement of originally dry particulates, and the degree to which these problems are solved, or remain unsolved, is the subject of ongoing debate in the art.
In view of the above, dry analysis of dry particulates, and the subsequent complete avoidance of the problems associated with the wet process particle analyzers, seems a desirable goal. Thus, many have labored to develop the other major category of particle analysis device, the dry process analyzer. For example, U.S. Pat. No. 3,269,189, of W. G. Monk, is believed to teach a device for classifying particulates by size and weight in a vacuum chamber by use of vibration and a controlled air or gas flow. At about the same time, the application of optical techniques to dry particle analysis was taught by U.S. Pat. No. 3,328,587, of T. J. A. Brown, et. al. The device of Brown maintains the particulate sample in a state of consolidation, and appears to rely for its operation only on back scatter of incident light from the sample. U.S. Pat. No. 4,563,581, of Perten discloses a later effort directed to an analyzer in which the particulate sample is also compacted in an analysis cell and back scatter alone apparently provides the available information about the sample.
U.S. Pat. No. 4,895,034, to Poole, is directed to an entirely different method of particle size analysis, that of aerodynamic and optical "time of flight" analysis. According to this teaching, particles are impelled in a cloud through a nozzle, and their size is optically measured by transit time across a known distance. Poole teaches to disburse particles as a cloud in a carrier stream of air using an air blast and agitation.
An effort directed to the analysis of particulates disbursed in a flow of air, which allows back scatter, as well as forward scatter or diffraction and obscuration techniques to be employed, is represented by the European patent application, publication number 0 144 018, having a publication date of 12 Jun. 1985. This latter effort provides an observation chamber which prepares a particulate sample in an air stream for optical analysis. The observation chamber itself relies, however, on an inflow of particulates, such as coal dust, already conveyed in an air stream. An ejector is employed to entrain additional ambient air, in addition to mixing compressed air with the particulate sample and its original conveying gas flow. How the particles are introduced from a bulk sample into the conveying gas stream is not detailed in this publication.
Another dry particle analysis device is know in the art, as depicted in a 1990 publication from Coulter Scientific Instruments, which is an operating company of Coulter Electronics, Inc. Overall, the described dry particle analyzer is an adaptation of a conventional particle analyzer previously used for liquid-borne samples. This publication briefly describes a bench-top dry particle module in which free-flowing particulates are fed from a vibratory sifter cup into the entrance of the module. The sifted particulate sample is believed to be conveyed with vacuum-induced ambient air flow in a flexible corrugated conveying tube, which tube lays generally horizontally on the bench top. The conveying tube connects to an analysis cell adapted to the conventional particle analyzer. On the laboratory bench top, the module and conveying tube are relatively large, and take up bench top space, which is always in short supply. Further, the conveying tube presents an opportunity for powder to settle out of the conveying air flow, and to coat the inside of the tube. Consequently, several questions are presented with respect to the integrity of the particulate sample when it arrives at the analysis cell. The air flow in the conveying tube would seem to be necessarily turbulent. In view of this turbulence, how can eddy currents be avoided in the tube, especially at bends in the tube? Eddy currents in the air and particle flow through the tube would likely cause relatively quiescent zones where particles could settle. Has part of the sample settled out in the conveying tube? If part of the sample does settle out in the tube, is it homogeneous to the remainder of the sample so that particle size distribution as measured is not affected? If the particulates do settle out of the conveying air flow, or plate the inside of tube because of adhesion, electrostatic attraction, or other causes, will this loss of sample be preferential to particular particle sizes?
Also, the possible settling of particulates in the conveying tube presents a cleaning problem. When the conveying tube is next used, one might reasonably question whether it was completely cleaned after the last use, or could remnants of the last test be contaminating the present test? The conveying tube is thought to be a durable component, rather than being disposable. Therefore, because the interior of the conveying tube is not accessible for cleaning other than by running a rag or bore brush through the tube, cleanliness of this tube would always seem to be in question. Also, the interior of the tube is not readily available for visual inspection of cleanliness. A glance down the tube would seem insufficient, and the use of a bore scope for visual inspection would seem way too time consuming. Thus, the dry powder module of Coulter would seem to have many disadvantages.
Still further, the vibratory feed system used by Coulter might interfere with the delicate optical system of the analyzer if these vibrations were allowed to reach the analysis cell. Audible noise from this vibratory particle feeder may also be an irritant to users of the device.